A previously known hydraulic pressure control apparatus of an automatic transmission includes an oil pan, which stores oil, a valve body, which includes a plurality of oil flow passages, and a solenoid valve (an electromagnetic valve), which forms a hydraulic circuit in cooperation with the oil flow passages of the valve body.
The electromagnetic valve is immersed together with the valve body into the oil received in the oil pan and includes valve elements (e.g., a spool valve and a ball valve) and a solenoid actuator (hereinafter referred to as a solenoid), which drives the valve elements (see for example, JP2009-180261A).
The solenoid includes a plunger, a coil, a stator core, a yoke and a connector. The plunger is driven in an axial direction by a magnetic force. The coil generates a magnetic flux around the coil when the coil is energized. The stator is configured into a tubular body and forms a magnetic path on a radially inner side of the coil. The yoke is configured into a tubular body having a bottom and forms a magnetic path on a radially outer side of the coil. The connector is for establishing an external connection and connects the coil to an external circuit(s).
The solenoid magnetically attracts the plunger to a magnetically attracting portion of the stator core with the magnetic force generated from the coil to drive the valve element in the axial direction.
JP2009-180261A discloses an electromagnetic valve, which includes a solenoid of a ring core type where a plunger directly slides in a guide hole of a stator core that is magnetized together with the plunger and the yoke by a magnetic force generated from a coil.
In order to limit sticking of the plunger caused by a volume change (breathing action) at a plunger receiving chamber that receives the plunger in a manner that enables reciprocation of the plunger at the time of operating the solenoid, a breathing passage is formed in the electromagnetic valve of JP2009-180261A. The breathing passage extends from an outside communication opening, which is communicated with an outside of the solenoid, to an inside communication opening, which is communicated with the plunger receiving chamber, to provide an oil supply path from the outside of the solenoid to the plunger receiving chamber.
The plunger receiving chamber includes two spaces, which are located on two opposite sides, respectively, of the plunger (i.e., a plunger front side space, which will be referred to as a first volume variable chamber, and a plunger back side space, which will be referred to as a second volume variable chamber).
In the solenoid of JP2009-180261A, foreign objects, which are contained in hydraulic fluid (oil) that inflows into and outflows from the second volume variable chamber through the breathing passage in response to a change in the volume of the second volume variable chamber, sink to a lower side in the breathing passage in the direction of the gravitational force at the time of reciprocating the foreign objects contained in the hydraulic fluid in the breathing passage, so that a majority of the foreign objects is removed from the oil that enters a distal end fluid chamber, and thereby intrusion of the foreign objects into the distal end fluid chamber is limited.
Foreign objects, such as metal debris particles (magnetic foreign objects made of iron or iron containing material, or contaminants) generated by, for example, friction of a transmission mechanism (a speed change mechanism) of an automatic transmission, are mixed in the oil, into which the electromagnetic valve is immersed together with the valve body. In a case where these foreign objects inflow along with the oil into the inside of the plunger receiving chamber through the breathing passage, when the foreign object(s) is caught in a slide clearance between the plunger and the stator core, smooth movement of the plunger relative to the stator core is interfered. Therefore, malfunction of the plunger may possibly occur.
Furthermore, in the solenoid of JP2009-180261A, a settling velocity (a sinking speed) of the minute foreign objects is slow. Therefore, when the plunger is moved before the minute foreign objects are settled, the minute foreign objects may possibly enter the second volume variable chamber.
In the case where the minute foreign objects enter the second volume variable chamber, the minute foreign object(s) may possibly be caught at a slide part (a slide clearance) between the outer peripheral surface of the plunger and the hole wall surface of the guide hole of the stator core.
When the minute foreign object(s) is caught in the slide clearance, smooth reciprocating movement of the plunger relative to the guide hole of the stator core may be interfered, so that an inoperable state (valve locked state) or a malfunctioning state of the plunger and a shaft connected to the plunger may possibly occur.